The invention relates to tandem machines with opposed free pistons comprising at least two groups of drive piston-cylinders arranged parallel beside one another and at least two compressor elements, each of these drive piston-cylinder groups comprising two aligned cylinders in each of which two opposed drive pistons work, the two outer drive pistons of each group being connected to compressor pistons through outer cross members fixed to one another by at least one linking element, so that an outer movable train is thus formed, whilst the two inner drive pistons of each group are connected together and to other compressor pistons to form thus an inner movable train.
In these machines the outer movable train is subjected to longitudinal vibrations created by the harmonics of the periodic forces as well as to bending vibrations created by dissymmetries of operation in the drive piston-cylinder groups, these dissymmetries being due particularly to differences in the combustion speed, in the combustion advance, in the state of the injectors, etc.
One of the parameters influencing the amplitude of these vibrations most unfavorably is the leverage length through which the compressor piston rods are connected to the abovesaid outer transverse parts (cruciform parts).
The invention is aimed notably at obtaining a shortening of these leverages and, consequently, a reduction in the amplitude of the abovementioned vibrations.
To this end, the compressor pistons forming part of the outer movable train are arranged outside the transverse parts of this train, whilst the drive pistons and the one or more linking elements forming part of this same movable train are on the inner side of said transverse parts.
In the case where each of the transverse parts of the outer movable train is mounted, so as to be slidable thereon, on a rod or a tube which extends between two fixed supports and which are to be found on each side of said part, it is advantageous to fix the cylinders of the compressor pistons forming part of this movable train on the outer support of said rod or of said tube.
To obtain high power, one is led to provide, on each side of the inner cross member, four inner drive pistons movable along the drive cylinders arranged at the corners of a rectangle and two compressor pistons sliding in the inner compressor cylinders whose axes pass outside said rectangle, the outer movable train comprising two outer cross members connected through at least one tie rod, each bearing four outer drive pistons movable in four of said drive cylinder and two compressor pistons sliding in outer compressor cylinders.
One of the problems which then occurs is constituted by the intrinsic mass between the inner and outer movable trains, which it is necessary to compensate by the addition of masses which cause considerable stresses on the synchronising means; another problem is constituted by the dimensions and the mass which the cross-members reach, which must transmit large and alternating forces between compressor pistons and drive pistons.
The invention is also aimed at reducing the mass of each of the trains in this case, by acting notably on that of the cross members which constitute an important element thereof, and at balancing these weights intrinsically as far as possible.
For this purpose, the axes of the inner compressor cylinders pass through the extension of the small median of said rectangle whilst the outer compressors, arranged outside said rectangle, pass through the extension of the large median of the latter.
One embodiment of the tandem machine according to the invention and of modifications are shown in the accompanying drawings.